1. Field of the Invention.
The present invention relates to electrolytic manganese dioxide for use as the cathode in batteries and methods of producing the electrolytic manganese dioxide. More particularly, the present invention provides electrolytic manganese dioxide which when utilized in batteries imparts higher discharge capacity at high discharge rates thereto.
2. Description of the Prior Art.
Electrolytic manganese dioxide, referred to in the industry as EMD, is widely used as the cathode material in batteries. EMD was first utilized in zinc-carbon cells (Leclanche cells), and later in alkaline cells.
EMD is used in alkaline batteries to achieve high voltages, low polarization and high discharge capacities. The discharge capacity of alkaline batteries is dependent upon the quality of the EMD utilized. The mixture forming the cathode in alkaline batteries is about 82% EMD and is generally formed into an annular cathode by impact extrusion in the container or by the compression molding of pellets which are recompacted against the container. In both of these processes, the properties of the EMD must be very consistent to allow the battery manufacturer to consistently produce high quality batteries. Alkaline batteries have a higher capacity per unit volume than other zinc-manganese dioxide batteries, and are particularly capable of high discharge capacity, i.e., long life, at high discharge rates.
To achieve the EMD purity required in its production utilizing the well known electrolysis method, a highly purified manganese sulfate solution must be provided to the electrolytic cell. In addition, the electrolytic cell must be operated within a narrow range of process conditions.
By the end of the 1980""s, EMD was developed having what was thought to be a satisfactory discharge capacity at the then maximum discharge rates demanded by portable devices, i.e., about 0.5 watt. However, during the past several years, the development of lap-top computers, video cameras, cellular phones and the like have brought about a demand for high discharge capacity at higher discharge rates, i.e., from 1 to 2 watts. While some improvements in battery performance at high discharge rates have been made, EMD has not changed significantly in discharge capacity or in the process conditions utilized in its production for many years. Thus, there is a continuing need for better, higher quality EMD whereby alkaline and other batteries utilizing the EMD have higher discharge capacities at the higher discharge rates presently required.
The present invention provides improved high quality EMD for use as cathode material in batteries of high discharge capacity at high discharge rates and methods of producing such EMD by electro-deposition in an electrolytic cell. The electrolytic cell includes cathodic and anodic electrodes disposed therein through which an electric current is passed. In accordance with the methods of the present invention, a heated aqueous electrolyte solution comprising sulfuric acid and manganese sulfate is maintained in the electrolytic cell. The solution is of high purity and includes sulfuric acid therein in the general amount in the range of from about 20 to about 60 grams of sulfuric acid per liter of solution and preferably in an amount in the range of from about 20 to about 50 grams per liter. Manganese sulfate is present in the solution in an amount whereby manganese ion is generally present in the range of from about 5 to about 30 grams of manganese ion per liter of solution and preferably in an amount in the range of from about 5 to about 25 grams per liter. The temperature of the electrolyte solution in the electrolytic cell is carefully maintained in the range of from about 95xc2x0 C. to about 98xc2x0 C. and the sulfuric acid in the aqueous electrolyte solution is maintained in an amount such that the ratio of the amount of sulfuric acid present to the amount of manganese ion present is greater than 2.0 but less than or equal to 4. An electric current is applied to the cathodic and anodic electrodes and the electrolyte solution whereby the anodic electrode current density is generally in the range of from about 2.5 to about 6 amperes per square foot and preferably in the range of from about 2.5 to about 4.5 amperes per square foot, and the high discharge capacity EMD produced is deposited on the anode.
The cathode utilized in the electrolytic cell is preferably comprised of copper, graphite or steel, as determined by cost. The anode is preferably comprised of titanium to provide minimum weight and volume, maximum strength, minimum weight loss, and adequate corrosion resistance.
It is, therefore, a general object of the present invention to provide improved electrolytic manganese dioxide having higher discharge capacity at high discharge rates and methods of producing the same.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.